This project is concerned with inductive functions between spinal motoneurons and the innervated sketetal muscles in mammals. It is known that fast conducting and fast discharging motoneurons innervate the fast contracting skeletal muscle, whereas the slowly contracting muscle is innervated by slowly conducting and slowly discharging motoneurons. Thus, there is a sort of matching in properties between motoneurons and muscles. When motoneuron properties were modified by partial denervation of the muscle or by transection of the spinal cord, alterations in motoneuron properties were found to be associated with changes in the contractile properties of the innervated muscle. It is concluded that the contractile properties of skeletal muscles are determined, at least in part, by the properties of the innervating motoneurons. In new-born kittens, on the other hand, there was no correlation in properties between motoneurons and muscles. During postnatal development, motoneurons and muscles appear to differentiate independently.